feat: rebrand logos with wordmark/mark variants and pipe-friendly API

- Replace old civilytics_logo.png/white.png/jpg/pdf with new rebrand
  assets: civilytics-wordmark.png, civilytics-wordmark-reverse.png,
  civilytics-mark.png, civilytics-mark-reverse.png (all transparent bg)
- make_logo_grob() now accepts type = c("wordmark", "mark") alongside
  variant = c("light", "dark") for 4 combinations
- New civilytics_logo() pipe-friendly convenience function:
    ggplot(df, aes(x, y)) + geom_point() + theme_civilytics() |>
      civilytics_logo()
  R's |> has lower precedence than +, so the full ggplot chain pipes
  through without parentheses
- Remove old icon/favicon assets no longer used
- Wrap showtext-dependent examples in \dontrun{} to prevent R CMD check
  failures in headless PostScript environments

man/theme_civilytics.Rd

@@ -64,6 +64,7 @@ automatically via [showtext] when the package is attached. Call
 [civilytics_load_fonts()] to reload them if needed.
 }
 \examples{
+\dontrun{
 library(ggplot2)
 ggplot(mpg, aes(displ, hwy)) +
   geom_point() +
@@ -74,3 +75,4 @@ ggplot(mpg, aes(displ, hwy, colour = class)) +
   scale_color_civilytics() +
   theme_civilytics()
 }
+}